BOOK IN SERIES
The type we are now going to describe should not be mistaken with the double staggered gearpumps . We can see in Figure 4.6 ( Single suction and delivery phases ) that the fluid flowing out of the upper outlet cannot be delivered uniformly . As a matter of fact , the oil held in each meatus is alternately driven by the right and left teeth , thus causing a pulsating delivery along with slightly variable pressures , noise and vibrations . Luckily , the cycle is not remarkably pulsating , so it does not have the trend shown in the left part of Figure 4.15 : as previously explained , the expulsion of the last drops from a specific tooth coincide with the beginning of the first phase of a tooth of the other wheel . Actually , the curve trend in standard pumps depends on the number of gear teeth : the more teeth a gear has , the less it pulsates ; yet , a high number of teeth cause different types of technical problems .
Q
Maximum �ow variation ( it does not occur in gear pumps )
Pulsating flow in gear pumps
t
Q
Figure 4.15
Standard �ow variation in standard gear pumps t
the other by a sheet with a clearance hole . Leading wheels are axial and solidly connected to a single transmission shaft and obviously the casing has a single outlet . The double pair of staggered gears reduces pulsations by about 70 % during delivery .
If noise is too loud and yet we want to use this very type of pump , we can resort to external helical gear pumps : they are quite noiseless but more expensive and they can sustain a lower nominal pressure than standard pumps because of the substantial loadsbetween the bushings and the teeth ( Figure 4.17 ).
A famous international company has recently launched a special gear pump that partially meets the need for variable flow . A single casing holds two separated external gear pumps that share delivery and suction hoses ; the parts are mutually axial and connected to the single transmission shaft . If the hydraulic circuit requires a limited flow , the flow is generated by a single pump , while the other pump is on standby due to its dedicated valve . When more flow ( hence more power ) is needed , a Load Sensing signal switches the valve of the second pump thus increasing the flow . However , this device cannot be classified as a variable flow pump because flow distribution , albeit automatically , is affected by the displacement of the two pumping parts : if both of them , at a certain rpm , pump 10 l / min , system flows can amount to as little as 10 or 20 l / min .
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| Global MDA Journal | NOV-DEC 2017
Figure 4.16
In double staggered gearpumps ( Figure 4.16 ), the teeth of the second pair of gears are offset by a half tooth spacing ( a tooth of the second pair is arrayed in the middle of the space between two teeth of the first one ) and divided from
Figure 4.17
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